An important step has been taken towards an era of virtually limitless energy from the nuclear reactions that power the sun.

Scientists at a fusion power facility in the US have succeeded in tipping the balance between energy input and output to achieve a net gain.

The result falls short of "ignition" - the "Holy Grail" of fusion science, which would generate more energy than the total amount needed to operate the reactor - but is still seen as a turning point.

A team at the National Ignition Facility (NIF) in California used 192 lasers to heat and compress a small pellet of fuel, a mixture of deuterium and tritium (DT), until its atoms began to collide and fuse.

Above: Instruments are viewed inside the target chamber at the National Ignition Facility

By carefully tuning the laser pulses to maintain stability, the scientists produced an energy yield 10 times greater than any achieved before.

Crucially, the fusion energy obtained exceeded the amount of energy absorbed by the fuel to trigger the reaction.

This is not the same as generating more than the total energy needed to compress the fuel pellet, which is necessary for ignition.

"There is more work to do and physics problems that need to be addressed before we get to the end," said lead scientist Dr Omar Hurricane, who led the research, reported in the journal Nature.

"But our team is working to address all the challenges, and that's what a scientific team thrives on."

Nuclear fusion powers the sun and other stars, and is also responsible for the devastating destructive force of a hydrogen bomb.

A peaceful application of the technology could provide almost limitless supplies of clean energy from special forms of hydrogen, the most abundant element in the universe.

Deuterium and tritium are two "heavy" isotopes, or atomic strains, of the gas.

Two different approaches towards fusion power are being tested by scientists. One confines the super-heated fuel in a magnetic field. The other, adopted by the NIF scientists, relies on high energy lasers to compress and heat the fuel until it implodes.

A key element of the "inertial confinement" laser technique is known as "boot strapping". This is when subatomic alpha particles produced by the reaction deposit their energy into the DT fuel, rather than escaping.

The particles further heat up the fuel, increasing the rate of fusion reactions and releasing more alpha particles. Eventually it is hoped the feedback effect will lead to ignition.

"What's really exciting is that we are seeing a steadily increasing contribution to the yield coming from the boot strapping process we call alpha particle self-heating as we push the implosion a little harder each time," said Dr Hurricane, from the US Department of Energy's Lawrence Livermore National Laboratory.

The latest experiments were designed to avoid a previous problem, the break-up of the plastic shell that surrounds and confines the DT pellet as it is compressed.

By modifying the laser pulses, the instability that caused the break-up was suppressed.

"We have waited 60 years to get close to controlled fusion, and we are now close in both magnetic and inertial confinement research. We must keep at it.

"The engineering milestone is when the whole plant produces more energy than it consumes."

The CCFE hosts the world's largest magnetic fusion experiment, JET (Joint European Torus). It is due to be superseded by the ITER (International Thermonuclear Experimental Reactor) now being built in France.